The present disclosure relates to a fixing device that fixes a toner image to a sheet and an image forming apparatus that includes a fixing device.
In an image forming apparatus that uses a toner to perform printing, a fixing device that pressurizes and heats a toner image transferred to a sheet is provided. The fixing device includes a heating rotation member that is heated by a heater and that makes contact with the sheet. The temperature of the heating rotation member is detected so that the heating rotation member is maintained at a temperature suitable for fixing and is prevented from being excessively heated. Here, a thermistor may be used for the temperature detection. The thermistor is provided for the heating rotation member and is connected to a resistor. A voltage between the thermistor and the resistor is taken out. The resistance value of the thermistor is varied with the temperature. The resistance value of the thermistor and a target temperature to be detected are determined according to the magnitude of the voltage taken out. Examples of the image forming apparatus having a circuit that performs temperature detection based on a voltage between a thermistor and a resistor include the followings.
Specifically, an image forming apparatus is known which has a fixing temperature detection circuit that includes a fixing thermistor and a fixed resistor and in which a connection point between the fixing thermistor and the fixed resistor is input to a print control portion 1 for performing AD conversion.
As described above, conventionally, a series circuit of a thermistor and a fixed resistor is provided, a voltage value between the thermistor and the fixed resistor is subjected to AD conversion and thus temperature detection is performed on a fixing device. An example of such a conventional circuit is shown in FIG. 14.
In the circuit of FIG. 14, for a series circuit 3000 of one thermistor 1000 and a fixed resistor 2000, one analogue-digital conversion circuit (AD conversion circuit) is needed. When an AD conversion circuit incorporating an integrated circuit such as a CPU or a microcomputer is used, a voltage between the thermistor 1000 and the resistor 2000 is input to an AD conversion port.
On the other hand, in order to perform temperature detection on both ends of a heating rotation member and the center position thereof, a plurality of thermistors may be provided in the heating rotation member. In an image forming apparatus, one or a plurality of thermistors for temperature detection may further be provided in other members within a fixing device or in places other than the fixing device.
However, when a plurality of thermistors are provided, an integrated circuit that has the same number of AD conversion ports as the number of thermistors that are provided needs to be adopted. Hence, disadvantageously, the choices of the integrated circuit to be adopted are limited. An integrated circuit having a large number of AD conversion ports may be expensive.
An AD conversion port of an integrated circuit may not be allocated to a thermistor. In such a case, an AD conversion circuit is provided separately from an integrated circuit. Consequently, it is difficult to reduce the manufacturing cost of an image forming apparatus. Even in the known image forming apparatus described above, the same number of AD conversion circuits and the same number of AD conversion ports as the number of thermistors that are provided are needed. Hence, it is impossible to solve the above problem.